This invention relates generally to heterojunction bipolar transistors (HBTs), and more particularly, the use of HBTs in an HBT array.
An HBT is a well known transistor configuration frequently used in power amplifier circuits. To increase the total power output of an HBT power amplifier, it has become commonplace to connect multiple HBTs in parallel to form a linear HBT array. The term xe2x80x9clinearxe2x80x9d is used to describe the side-by-side, uniformly aligned arrangement of the HBTs in the array.
The thermal design of an HBT array is critical to reliable operation of the device in which it is used. Several factors contribute to increased temperatures during the operation of the HBT array. An HBT array can be described generally as comprising active regions (the region comprising the HBTs themselves) and inactive or xe2x80x9cprotectivexe2x80x9d regions (the areas surrounding the HBTs; essentially the entire array except for the HBTs). The flow of current across the base emitter junction of an HBT dissipates power in the form of heat, causing an increase in the temperature in and around the junction. Elevated junction temperature not only degrades power and gain performance, but it also reduces the device lifetime.
When several HBTs are arranged in a linear array, additional thermal issues arise. HBTs generate heat during normal operation, and that heat is transferred, by thermal coupling, to the inactive region around the HBT. If adjacent HBTs are close enough together, the heat of one HBT can also be transferred to the adjacent HBT(s), causing their temperature to rise. Further, parallel HBTs, although theoretically identical, are typically not truly identical and thus do not have identical heat dissipation capability, resulting in the possibility of having two xe2x80x9cidenticalxe2x80x9d HBTs that operate at different operating temperatures.
If one HBT in an HBT array operates even slightly hotter than an adjacent HBT, the hotter HBT begins to carry more current, which results in a further increase in its temperature, which further increases the current it carries. This leads to a runaway condition (known as xe2x80x9cthermal runawayxe2x80x9d) whereby the temperature increases to the point of destruction of the hotter-operating HBT. Once that HBT is destroyed, the remaining HBTs must carry the current load of the destroyed HBT, thereby causing their temperatures to increase as well. Eventually, the entire device will fail.
A generally accepted way of controlling thermal runaway problems in linear HBT arrays is to place a fixed resistance in series with each emitter (referred to as xe2x80x9cemitter-ballastingxe2x80x9d) or each base (referred to as xe2x80x9cbase-ballastingxe2x80x9d) of the transistors that are connected in parallel. As is well known, the use of ballasting resistors forces essentially uniform current distribution among the HBTs, thus reducing the likelihood of the occurrence of a runaway condition caused by a lack of uniformity in the heat dissipation characteristic of the HBTs in the array. Examples of emitter-ballasting can be found in, for example, U.S. Pat. No. 6,130,471 to Boles and U.S. Pat. No. 5,378,922 to Sovero, all of which are incorporated herein fully by reference. Examples of base-ballasting can be found in U.S. Pat. No. 5,321,279 to Khatibzadeh et al., U.S. Pat. No. 5,629,648 to Pratt, U.S. Pat. No. 5,608,353 to Pratt, and U.S. Pat. No. 5,760,457 to Mitsui et al., also incorporated herein fully by reference.
To deal with the thermal coupling issue (as well as to prevent collector-to-collector leakage current), designers must leave a minimum amount of space between each HBT in a linear HBT array to increase the size of the inactive region between adjacent HBTs. The typical pitch between HBTs (the distance from the center of one HBT cell to the center of another adjacent cell) is 40-50 xcexcm. This minimizes the heat transferred from one HBT to another. While this helps reduce the impact of thermal coupling on the operation of the device, it restricts the designer""s ability to reduce the overall size of the linear HBT array and thus any components that incorporate linear HBT arrays, since they cannot move the HBTs closer to each other without lowering the performance of the array or violating the collector-to-collector leakage isolation requirements.
Accordingly, it would be desirable to have an HBT array in which the HBT devices could be compacted closer together to achieve a smaller overall device size without increasing the likelihood of damage and/or reduced performance due to thermal coupling, thermal runaway and/or collector-to-collector leakage.
In accordance with the present invention, HBTs in an HBT array are configured non-linearly, i.e., staggered, thus reducing the impact of thermal coupling between adjacent HBTs in the array and bypassing the minimum collector-to-collector spacing design rules required for a linear HBT array. Using this non-linear configuration, adjacent HBTs are misaligned with respect to each other, rather than being in the aligned, directly side-to-side arrangement of the prior art. In a preferred embodiment, adjacent HBTs in the array are configured in a corner-to-corner arrangement, and in a more preferred embodiment, the collectors of the adjacent HBTs are aligned or are common, i.e., the collector of one HBT is shared with the collector of an adjacent HBT. In a most preferred embodiment, the HBTs are ballasted in an emitter-ballast/base-ballast pattern (referred to as xe2x80x9cmixed ballastingxe2x80x9d or xe2x80x9cdual-ballastingxe2x80x9d). (The result is a more compact array of HBTs that exhibit the lower temperature characteristics of ballasted HBTs while also demonstrating a reduced thermal coupling effect on adjacent HBTs.)
Power amplifiers fabricated using the preferred dual ballasted nonlinear HBT arrays are significantly smaller than the typical power amplifiers, with no degradation in thermal resistance or operating junction temperatures.